Time-Resolved Impulsive Stimulated Raman Studies of 1,1‘-Binaphthyl in the Excited State: Low-Frequency Vibrations and Conformational Relaxation

Abstract

The conformational relaxation of excited-state 1,1‘-binaphthyl in solution was investigated by using femtosecond time-resolved absorption and time-resolved time-domain Raman spectroscopy. Femtosecond time-resolved absorption measurements confirmed that the first transient (the “unrelaxed” S1 state) is generated in a subpicosecond time scale after photoexcitation and that it undergoes a relaxation with a time constant of 16 ps (in cyclohexane) to form the second transient (the “relaxed” S1 state). Time-domain Raman experiments provided low-frequency Raman data of the relaxed S1 state for the frequency region of 0−400 cm-1. The obtained Raman data of the relaxed S1 state were significantly different from the low-frequency S0 spectrum in solution. The relaxed S1 spectrum resembles the spectrum of the crystalline form that has a dihedral angle of 69°, whereas the S0 Raman spectrum in solution coincides well with that of the crystalline form having a dihedral angle of 103°. The time-domain Raman measurements provided vibrational data that support a conformational change occurring in 1,1‘-binaphthyl in the S1 state.